Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity

Pallas-Sanz, E, Johnston TMS, Rudnick DL.  2010.  Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity. Journal of Geophysical Research-Oceans. 115

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3-dimensional circulation, confluent flow, frontogenesis, motion, ocean fronts, omega-equation, potential vorticity, q-vector, secondary circulation, semi-geostrophic equations


This is the second paper investigating the three-dimensional dynamics from two consecutive, quasi-synoptic surveys of a shallow front in the California Current System. The mesoscale vertical velocity (w) is obtained by solving a generalized omega equation using density and horizontal velocity observations. Highly nonlinear dynamics emerge from the ageostrophic forcing terms for w in an adiabatic generalized omega equation. The two main processes driving w are (1) wind-induced cross-frontal ageostrophic circulation (survey 1) and (2) ageostrophic kinematic deformation during frontogenesis (surveys 1 and 2). The horizontally averaged heat fluxes are positive in the whole water column with maxima at similar to 50 m, which warms (cools) the upper (lower) water column and upwells (downwells) light (dense) water. Wind-induced currents interact with the front, cooling the upper ocean and creating a divergent potential vorticity (PV) flux at the Ekman layer base which weakens the vertical heat and PV fluxes in survey 1. Vertical velocity extrema reach similar to 10 m d(-1) in both surveys. A diabatic omega equation is derived, which introduces an important new idea: the relation of the frictional w with the vertical diffusivity of the differential ageostrophic vorticity. This term is not found in the quasi-geostrophic omega equation. By including vertical mixing, |w| is enhanced by a factor of 2 in the upper similar to 100 m and reduced below. This effect is pronounced when the wind blows in the direction of the frontal jet, but it is sensitive to the vertical mixing parameterization.






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